The construction of boats of fibreglass has usually involved the building of an inner form or male mould. The surface of this inner form was then highly polished, and a female mould, i.e., an outer form was then constructed on the inner form. This outer form was usually constructed in two longitudinal halves, and when assembled, the boat was then built by laying up successive layers of glass material, the outermost layer of the article being in fact in contact with the inside surface of the mould. In this way, the boat was constructed with a perfectly smooth exterior surface, and with an interior surface which was somewhat rough and unfinished. However, usually this interior roughness did not matter since it would be concealed.
The construction of an inner form followed by the construction of an outer mould is an extremely expensive and time consuming operation and greatly increases the construction costs of such articles.
New glass reinforced building techniques have been developed in which the boat was built from the inside out. In this form of construction all that is required is to build the inner form, and then start laying up the glass material from the inside of the boat hull. In this way, it was unnecessary to provide a very high degree of finish on the surface of the inner form, since the inner surface of the hull would in any event be concealed. The use of this technique resulted in the development of new construction materials and techniques including so-called "sandwich" materials such as plastic foam sheet, and balsa wood and the development of a suitable form of material or filler for forming a smooth exterior surface such as talc, or a specially developed filler material known as "micro-balloons." By the use of these techniques, it is possible to build a boat having a glass reinforced inner shell, an intermediate layer of sandwich material such as plastic foam or balsa wood and an outer glass layer, the outer glass layer being finished with a layer of resin material and filler such as talc or micro-balloons which could then be sanded down very easily to a satisfactory finish.
However, even this form of construction, while being a great improvement over the old form of construction, still requires the production of an accurate inner form which is usually built of wood.
As a result of the requirement for building the inner form, it is usually found that these improved techniques are not available to the average home boat builder. In most cases, he cannot build the inner form himself, and it is uneconomical for a manufacturer to build the inner plug and supply it to the home builder. As a result, the usual practice in the trade is to build the boat hulls in a factory, and a hull will then either be finished in the factory, or shipped to the new owner's home where he may finish it himself. In either case, the finishing costs or the shipping costs are very considerable and greatly increase the basic cost of the hull, and the factory labour portion of the cost of the hull itself also adds substantially to the end price.
It is therefore desirable that some means should be provided whereby a home boat builder can actually erect the inner plug or mould in his home and then build the hull on it. In this way, the greatest degree of economy is achieved, leading to increased sales of glass and resin and sandwich core materials, and a wider interest in boating, and the home building of boats, and a more profitable utilisation of the leisure time of the individual home builder.
Portable moulds or forms for hollow articles or structures have been proposed in the past, usually based on the principle of an inflated body of some kind. Such inflatable moulds have been relatively crude and clumsy, however, and suitable only for building structures having simple, regular geometric contours. Such prior systems have not been suitable for the contruction of boats, since boat hulls have complex convex and concave compound curves, that must be built to close tolerances, in order to perform in accordance with the specifications of the designer.